We are continuing to study the basis for the energization of membrane transport systems for amino acids, and the structural differences, both in the transported substrates and complementarily in the transport receptor sites, that divide that transport among separate transport-mediating systems. In particular, attention is directed to the uphill operation of System L and need for an energy input not supplied by gradients of Na ion or of other amino acds. Incomplete evidence suggests a contribution by oxidation-reduction activity in the plasmid membrane. The rat hepatocyte, either freshly isolated or in primary culture, is being used to look for new Na ion-dependent transport systems by extended characterization of known systems in search for undetected heterogeneity. Discovery of a second transport system showing adaptive regulation but unresponsive to insulin and glucagon shows us that transport regulation and interaction between transport systems is still incompletely described. An anomaly in the stereoselectivity of a system for anionic amino acid applies so far to aspartate only, and not to glutamate or the model substrate, cysteate. Transport activity for cationic amino acids is under study both in the hepatocyte and in cultured skin fibroblasts.